Forest biodiversity: lessons from history for conservation.

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Chapter 12 (Page no: 163)

Many regions of Europe and eastern North America share broadly similar histories of land use over the past several centuries, with varying degrees of forest clearance followed by varying degrees of forest recovery. Here I use metapopulation models to explore the influence of the extent and timing of forest clearance and recovery on patch occupancy of slow-colonizing forest herbs in ancient and recent forests. If forest clearance exceeds the threshold for metapopulation persistence, extinction may require centuries to occur even when clearance is most severe. This suggests a heavy extinction debt for forest herbs in fragmented landscapes. The time between forest clearance and recovery (tc-r) can have a strong influence on metapopulation dynamics. If tc-r is short, patch occupancy in ancient forests (PAO) will start out high, and exert a strong 'colonization pressure' on recent forests, increasing patch occupancy in recent forests (PRO) above its equilibrium value before it ultimately declines towards equilibrium. If tc-r is long, PAO starts out low, and both PAO and PRO rise slowly towards equilibrium. The time required for PRO to reach equilibrium is generally in the order of centuries, and increases with tc-r. The results indicate that there may be long periods of time (decades to centuries) during which patch occupancy increases or decreases even in the absence of directional environmental change.